Han Seo Cho

PCB-compatible optical interconnection using 45/spl deg/-ended connection rods and via-holed waveguides

In this paper, a new architecture for a chip-to-chip optical interconnection system is demonstrated that can be applied in a waveguide-embedded optical printed circuit board (PCB). The experiment used 45/spl deg/-ended optical connection rods as a medium to guide light paths perpendicularly between vertical-cavity surface-emitting lasers (VCSELs), or photodiodes (PDs) and a waveguide. A polymer film of multimode waveguides with cores of 100/spl times/65 /spl mu/m was sandwiched between conventional PCBs. Via holes were made with a diameter of about 140 /spl mu/m by CO/sub 2/-laser drilling through the PCB and the waveguide. Optical connection rods were made of a multimode silica fiber ribbon segment with a core diameter of 62.5 and 100 /spl mu/m. One end of the fiber segment was cut 45/spl deg/ and the other end 90/spl deg/ by a mechanical polishing method. These fiber rods were inserted into the via holes formed in the PCB, adjusting the insertion depth to locate the 45/spl deg/ end of rods near the waveguide cores. From this interconnection system, a total coupling efficiency of about -8 dB was achieved between VCSELs and PDs through connection rods and a 2.5 Gb/s /spl times/ 12-ch data link demonstrated through waveguides with a channel pitch of 250 /spl mu/m in the optical PCB.

Passively assembled optical interconnection system based on an optical printed-circuit board

We propose a passively assembled chip-to-chip optical interconnection system using fiber-optic technology. To demonstrate the system, three components were prepared: a fiber-embedded optical printed-circuit board (OPCB), optical transmitter/receiver modules, and 90/spl deg/-bent fiber connectors. All components were assembled using precise guide pins and holes so that complete passive alignment was achieved in the OPCB. An optical link of 5-Gb/s/ch signals with a total link loss of -1.5 dB has been successfully demonstrated from the assembled system.

High-coupling-efficiency optical interconnection using a 90/spl deg/-bent fiber array connector in optical printed circuit boards

A high-coupling-efficiency optical interconnection has been demonstrated using a 90/spl deg/-bent fiber array connector to deflect beams between surface-emitting lasers or surface-receiving photodiodes and optical layers embedded in a board. A 90/spl deg/-bent fiber array is mounted in a tetragonal body with a millimeter scale size to make it suitable for passive packaging in the board. The bending radius of silica fibers in the connector was controlled to have 1.5 mm resulting in bending loss of about 0.5 dB. An optical link of 2.5-Gb/s signals with a total interconnection loss of -1.3 dB was demonstrated using the connectors and a fiber-embedded board.

Compact packaging of optical and electronic components for on-board optical interconnects

An optical interconnection plate was developed in order to achieve a compact and cost-effective interconnection module for an optical data link between chips on printed circuit boards. On the silica substrate, transmission lines and solder bumps are formed on the top surface of the substrate, and polymer waveguide array with 45/spl deg/ mirror planes is formed on the back side. This optical interconnection plate technique makes the alignment procedure quite simple and economical, because all the alignment steps between the optical components can be achieved in wafer processes and a high accuracy flip-chip bonding technique. We confirmed the sufficiently high coupling efficiency and low optical crosstalk using the simplified experimental setup. Flip-chip bonding of the vertical-cavity surface-emitting laser and photodiode arrays on the top surface of the optical interconnection plate was performed using indium bumps in order to avoid thermal damage of the polymer waveguide. The fully packaged optical interconnection plate showed an optical data link at rates of 455 Mb/s. Improvement of the mirror surface roughness and the mirror angle accuracy could lead to an optical link at higher rates. In addition, the interconnection system can be easily constructed by inserting the optical interconnection plate between the processing chips or data lines requiring optical links.

Parallel optical transmitter module using angled fibers and a V-grooved silicon optical bench for VCSEL array

We propose an advanced structure of optical subassembly (OSA) for packaging of the vertical-cavity surface-emitting laser (VCSEL) array, using (111) facet mirror of the V-groove ends formed in a silicon optical bench (SiOB) and angled fiber apertures. The feature of our OSA can provide a low optical crosstalk between neighboring channels, a low feedback reflection, and a large misalignment tolerance along the V-groove. We describe the optimized design of fiber angle, VCSEL position, and fiber position. The fabricated OSA structure consists of 12 channels of angled fiber array, 54.7deg V-grooves, Au-coated mirrors on (111) end facet of the V-grooves, and flip-chip-bonded VCSEL array on a SiOB. In this structure, the beam emitted from the VCSEL is deflected at the 54.7deg mirror of (111) end facet and propagated into the angled fiber. The angled fiber array was polished by 57deg. Fabricated OSAs showed a coupling efficiency of 30%-50% that is 25 times larger than that obtained from an OSA with a vertically flat fiber array. Our OSA showed large misalignment tolerance of about 90 mum along the longitudinal direction in the V-groove. We fabricated a parallel optical transmitter module using the OSA and demonstrated 12 channels times2.5 Gb/s data transmission with a clear eye diagram

VCSEL array module using (111) facet mirrors of a V-grooved silicon optical bench and angled fibers

We propose an advanced scheme of optical subassembly (OSA) using a silicon optical bench (SiOB) for the vertical-cavity surface-emitting laser (VCSEL) array. The VCSEL beams were deflected on the (111) end facets of the V-grooves in a SiOB and were coupled into the angled fibers. The inclined angle and position of the fibers were designed to maximize the coupling efficiency. The fabricated OSA showed a coupling efficiency of 30%-50% and a large misalignment tolerance of about 90 /spl mu/m along the longitudinal direction of the V-grooves. Data transmission of 2.5 Gb/s /spl times/12 channels was demonstrated with clear eye diagrams.

Optical interconnection using fiber-embedded boards and connection blocks fabricated by a micro-grooving technique for fiber insertion

We fabricated fiber-embedded boards using a micro-grooving technique for optical interconnects. This approach is quite cost effective and fully compatible with conventional PCB processes. U-shaped grooves were formed on FR-4 plates using a 90? V-shaped diamond blade. Polyimide-coated glass fibers were inserted in the grooves followed by a conventional lamination process. The 12 fibers embedded in the board showed a good uniformity in their position with a fluctuation below ?8 ?m. Optical connection blocks were also fabricated using fiber segments embedded in a grooved silicon wafer piece in order to couple the light between a transmitter/receiver module and a fiber-embedded board. 45? mirrors were formed at the end of the connection blocks by mechanical polishing. An optical link of 2.5 Gb s?1 signals with a high coupling efficiency was demonstrated through the fiber-embedded connection blocks and a board.

Optoelectronic and microwave transmission characteristics of indium solder bumps for low-temperature flip-chip applications

This paper describes low-temperature flip-chip bonding for both optical interconnect and microwave applications. Vertical-cavity surface-emitting laser (VCSEL) arrays were flip-chip bonded onto a fused silica substrate to investigate the optoelectronic characteristics. To achieve low-temperature flip-chip bonding, indium solder bumps were used, which had a low melting temperature of 156.7degC. The current-voltage (I-V) and light-current (L-I) characteristics of the flip-chip bonded VCSEL arrays were improved by Ag coating on the indium bump. The I-V and L-I curves indicate that optical and electrical performances of Ag-coated indium bumps are superior to those of uncoated indium solder bumps. The microwave characteristics of the solder bumps were investigated by using a flip-chip-bonded coplanar waveguide (CPW) structure and by measuring the scattering parameter with an on-wafer probe station for the frequency range up to 40 GHz. The indium solder bumps, either with or without the Ag coating, provided good microwave characteristics and retained the original characteristic of the CPW signal lines without degradation of the insertion and return losses by the solder bumps

Fabrication of fiber-embedded boards using grooving technique for optical interconnection applications

A simple method for fabricating fiber-embedded boards using a grooving technique is described that is quite cost effective and fully compatible with conventional printed circuit board (PCB) processes with no necessity for a specially designed wiring machine. FR-4 plates are grooved using a dicing saw machine and followed by placing optical fibers into the grooves. The fiber-embedded PCBs are laminated by conventional PCB processes at a temperature of 180°C for 1 h under 47 kg/cm 2 of pressure. The 50/125-µm glass fibers, and the polyimide-coated glass fibers are laminated successfully. In the fiber-embedded boards with a length of 10 cm, the variation of center positions of the embedded glass fibers is about ±5 µm. The transmitted optical power through the fiber-embedded boards shows a good uniformity of less than ±0.5 dB variation from the average value for the 12 fiber channels. Data transmission through the board at data rates of 2.5 Gbits/s is achieved. After confirming the successful laminations and the data transmission with the small-scale fiber-embedded boards, a large-scale prototype of the fiber-embedded board for a backplane application is successfully fabricated.

Characteristics of Indium Bump for Flip-Chip Bonding Used in Polymeric-Waveguide-Integrated Optical Interconnection Systems

We have flip-chip-bonded vertical-cavity surface-emitting laser (VCSEL) arrays on polymeric-waveguide-integrated optical interconnection systems. Using indium solder bumps, thermal damage to the polymeric waveguide can be avoided. Fracture occurs between the indium solder bumps and the VCSEL chip pad during the die shear test. It is inferred that both the low bonding temperature and the oxide layer formed on the surface of the indium solder prevent the bump from interacting with the chip pad. To reveal the microstructures of the joints between the bump and the chip pad, several specimens are cut into cross sections and polished. Scanning electron microscopy (SEM) with an energy dispersive X-ray (EDX) spectroscopic system is used to examine the microstructures and analyze the element compositions. Also, the optoelectronic characteristics of VCSEL arrays that were flip-chip-bonded under different bonding conditions are compared by current-voltage (I-V) and light-current (L-I) inspection.